This invention relates to reducing the flow induced vibration of a center body member within a conduit having an abrupt increase in cross section, and in particular to reducing the vibration of a nuclear reactor control rod in its guide tube.
The fission rate and thus the heat generation in modern power reactors is usually controlled by the insertion and removal of control rods into or between fuel assemblies in the reactor core. Particularly in reactors of the pressurized-water type, cylindrical control rods are reciprocated within guide tubes which are part of the fuel assembly. The fuel assemblies are subjected to the flow of primary coolant in order to remove the heat generated in the fuel. When in the core, control rods also produce heat through the nuclear transformation associated with their high neutron absorption rate, and so the control rods must also be cooled. Thus a requirement is imposed on the design of the guide tube for assuring that a minimum flow rate through the guide tubes will exist at all times.
During power production most of the control rods are maintained in a withdrawn position above the core. The lower tips of the rods, however, are not completely withdrawn from the guide tubes. Recent operating experience has shown that rodded fuel assemblies that have been in an operating reactor for a period of time have significant wear on the inner walls of the guide tubes at precisely the elevation corresponding to the withdrawn control rod tips.
It is known that under some conditions a self-excited vibration of a blade-type control element can occur when the insertion of the blade into a narrow upstream section of the flow path between fuel assemblies is less than a critical distance. Also, the fluid flow rate through the path must exceed a critical value for this vibration to occur. Suggestions have been made for reducing these vibrations by inserting labyrinth-type flow restrictions at various locations along the flow path downstream of the leading end of the blade, or by introducing a mechanical restraint to provide a lateral force which prevents the build up of small random vibrations.
Flow induced periodic vibrations have also been observed in arrangements having a cylindrical rod eccentrically located in an annular diffuser. It is believed that two kinds of vortices interact as follows to produce the periodic driving force. When the rod is centered in the tube, a diffuser vortex ring forms around the rod (like a donut) in the diffuser region as the flow separates upon exiting the guide tube. If the rod becomes eccentric in the tube, axial vortices, also called secondary flows, originate near the control rod tip and travel along the rod before entering the diffuser region where vortex bursting occurs. The interaction in the diffuser region of the bursting axial vortices with the diffuser vortex ring produces an oscillating driving force on the control rod.
It has been proposed to provide a circumferential fence in the flow path downstream of the diffuser, or to provide strakes in the outer wall of the diffuser starting at the diffuser mouth and extending longitudinally downstream of the diffuser. These solutions are not practical for use in nuclear reactors because the diffuser region can be very large.
Since nuclear reactor control rods often are not exactly centered in their guide tubes, the rod tip has a tendency to assume a rest position against the inner wall of the tube. Any significant tip vibration against the inside of the guide tube could produce wear on the inner wall and ultimately perforate the guide tube. It has been found that, except for significantly reducing the mass flow rate in the guide tube, the above suggested remedies for reducing flow induced vibration of a control rod are only marginally effective. If the flow rate is reduced sufficiently to eliminate vibration, it is often not possible to adequately cool the control rod.